Magnetic recording disk drives that use “shingled writing”, also called “shingled recording”, have been proposed. In shingled writing, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. The narrower data tracks thus allow for increased data density. The data tracks are arranged on the disk as annular bands separated by annular inter-band gaps. When data is to be re-written, all of the data tracks in an annular band are also re-written. Shingled writing is well-known in the art, for example as described in U.S. Pat. No. 6,185,063 B1.
In magnetic recording disk drives the magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data regions that define the data “bits” are written precisely and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (Ku) are required. The thermal stability of a magnetic grain is to a large extent determined by KuV, where V is the volume of the magnetic grain. Thus a recording layer with a high Ku is important for thermal stability. However, increasing Ku also increases the short-time switching field H0 of the media, which is the field required to reverse the magnetization direction. For most magnetic materials H0 is substantially greater, for example about 1.5 to 2 times greater, than the coercive field or coercivity Hc measured on much longer time-scales. Obviously, the switching field cannot exceed the write field capability of the recording head, which currently is limited to about 12 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted recording (TAR), also called heat-assisted magnetic recording (HAMR), wherein the magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed TAR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a conventional magnetoresistive (MR) read head.
Some proposed TAR disk drives use a “wide-area” heater that heats an area of the disk much wider than the data tracks. A wide-area heater, typically a waveguide coupled to a laser and with an output end near the media, is relatively easier to fabricate and implement in a conventional recording head structure. The previously-cited related application discloses a shingled-recording TAR disk drive with a wide-area heater.
In a TAR disk drive with a wide-area heater, the wide-area heater will heat data tracks in bands adjacent to the band being re-written. Wide-area heaters have been shown to result in substantial adjacent track erasure (ATE) because the peak temperature extends into adjacent tracks. Because the data tracks adjacent to the data track being written are also heated, the stray magnetic field from the write head may erase data previously recorded in the adjacent tracks. Moreover, even in the absence of a magnetic field, the heating of adjacent data tracks will accelerate the thermal decay rate of the media in adjacent tracks over that at ambient temperature, leading to possible ATE due to thermal effects alone. ATE generally translates into an increase in bit error rate (BER), resulting in degradation of the performance of the disk drive. In some severe cases, poor BER will lead to a significant increase of unrecoverable data errors. ATE has been described by Zhihao Li et al., “Adjacent Track Erasure Analysis and Modeling at High Track Density”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003, pp. 2627-2629.
Thus in a shingled-recording TAR disk drive with a wide-area heater it is necessary to avoid ATE of tracks in the bands adjacent to the band where data is being written.